| Along with the Cr (VI) is as carcinogenic, more and more people pay high attention to Cr (VI) in leather. Studies have shown the existence of the micro Cr (VI) in leather come from the oxidation of Cr (Ⅲ) by the synergistic effect of the oxidation process of unsaturated fatliquoring. As a temporary irreplaceable chrome tanning, how to solve the high Cr(VI) content has become a difficult problem in leather industry.Based on extensive research, we found the hydroxybenzene have inhibitory effects on these oxidations. our team study on the antioxidant of hydroxybenzene and its prevention on Cr(VI) in leather deeply, synthesize a special agent to prevent Cr(VI) and address the issue of control hexavalent chromium additives, and apply it in leather.Our studies include:Breaking down the oxidation of unsaturated oil by hydroxybenzene so as to prevent Cr(Ⅲ) from oxidizing into Cr(VI)was investigated. PVC sponge was used as a reacting carrier, taking the advantage of the sponge's porous character similarly to the leather. The oleic acid, chrome liquid and different phenols such as pyrocatechin, hydroquinone, gallic acid and pyrogallicoleic acid were absorbed by sponge and then put in box with invariableness temperature and humidity to oxidize deeply. The iodine value, peroxide value of oils and the contents of the Cr(VI) were determined in regular intervals also. The structure change of oleic acid was analysis by IR and GC-MS at different time. The results showed that the mono cyclic polyhydroxy phenol has inhibitory effect on the oxidation of oleic acid caused byα-H in both speed and output and also on oxidation of double bound synchronously. The different mono cyclic polyhydroxy phenol has the different inhibitory effect on the oxidation of the oleic acid. In generally speaking, the ability to inhibit oxidation ranks decreasingly as follow: trihydroxy phenol, dihydroxy phenol, single hydroxyl phenol, when with same number of hydroxyl, the o-hydroxy phenol, the p-hydroxy phenol, but when the carboxyl, the electron-absorbing group, opposite to the hydroxyl, the ability of polyhydroxy phenol with one ring in oxidation resistance would be weaken. The oxidation possibility of Cr(Ⅲ) into Cr(VI) is closely related to peroxide value. The higher the peroxide value is, the much the quantity of Cr(Ⅲ) oxidized to Cr(VI) is, and vice versa. When the system contains phenol the peroxide of unsaturated lipid could be inhibited and resulted in inhibition of oxidation of Cr(Ⅲ) into Cr(VI). The hydroxyl radical is the main free radical in the effect on Cr(VI) formation by free radicals after lipid oxidation. And the hydroxyl radicals are caused by a series of radical reaction fromα-H autoxidation. Therefore, control activity ofα-H determine the Cr (VI) formation, and the clearance rate of hydroxyl radical is an important indicator on the final Cr (VI) prevention.Base on the study on the monocyclic polyhydroxy phenol with one ring, our team study further on the polycyclic polyhydroxy phenol. Vegetable extracts, the Valonia, Tara, and the main components of vegetable extracts, the gallic acid, ellagic acid, as test samples are studied to explore the relationship between reducibility antioxidation of these materials and ability to prevent Cr(Ⅲ) from oxidation into Cr(VI).Abilities of these materials to eliminate free radical were measured by the following tests: Given quantities of materials above on the base of equivalent phenolic value were taken to ascertain reducibility by absorbency method of K3Fe(CN)6 reduce, eliminating ability against hydroxyl free radical by absorbency method of pyrogallol-red fading, eliminating ability against superoxide anion free radical by the absorbency method of pyrogallol autoxidation. These parameters were considered as the index of ability to prevent Cr(Ⅲ) from being oxidized into Cr(VI).The prevention of samples to Cr(Ⅲ) from being oxidized into Cr(VI) was measured. PVC sponge was used as a reacting carrier, the oleic acid, chrome liquid and different phenols were absorbed by sponge and then put in thermostat/ humidistat at 55℃and 17% relative humidity for 14 days with interval determination the quantity of Cr(VI) against one piece of sponge in groups by first time after two days and then every three days.Main results: Gallic acid, Tara, ellagic acid and Valonia all have preventive effects on the oxidation of Cr(Ⅲ) in leather because of its active phenolic hydroxyls which have reducibility, antioxidation, the clearance ability of DPPH radical, hydroxyl radical, superoxide anion radical, lipid peroxy radical. The effect to prevent Cr(Ⅲ) from Cr(VI) by vegetable extracts such as members of gallic acid family is closely related to their reducibility, antioxidation and also abilityb of complex with Cr(Ⅲ). All of the four samples, ellagic acid system is best in preventing Cr(Ⅲ) from Cr(VI),which is attributed to stronger antioxidation and reducibility, also to greater ability of complex with Cr(Ⅲ). The mechanism to prevent Cr(Ⅲ) from Cr(VI) by vegetable extracts such as members of gallic acid family is forming semi-quinone structure of ortho-phenolic hydroxyl, releasing the free hydrogen, capturing free radicals and meanwhile complexing with Cr(Ⅲ), forming chelates to reduce free Cr(Ⅲ) in the system. Based on study of mechanism above, when the chemicals with ability to prevent Cr(Ⅲ) from Cr(VI) is designed it should be taken into consideration: reducibility, antioxidation, the complexation with Cr(Ⅲ).After ellagic acid was selected as the best antioxidant, our team studied on the effect of gelatin on the clearance ability of ellagic acid against hydroxyl radical, DPPH radical, superoxide anion radical, lipid peroxy radicals. The experiment used PVC sponge as leather environment, oleic acid as fatliquoring, gelatin as collagen fibers. The sponge with chromium solution, oleic acid, ellagic acid was taken in aging test to optimize the adding amount of ellagic acid and study the complexation of ellagic acid, free Cr(VI) and gelation.The results showed that: ellagic acid has a strong clearance ability to hydroxyl radical, superoxide anion radical, lipid peroxy radical and also the prevention of Cr(VI) formation. Collagen itself has certain clearance effect on hydroxyl radical, lipid peroxy radical through the complex with metal ions. When the Ellagic acid and collagen proportional mix, the clearance effect on hydroxyl radical, superoxide anion radical, lipid peroxy radicals will be greatly weakened, and also the prevention of Cr(VI) formation.In view of ellagic acid is expensive, makes the leather green after excessive using, causes the problem in dyeing, has poor water solubility and low absorption in the leather processing, has poor compatibility with fatliquoring and can not be dispersed in oil largely which is disadvantage to its long-term antioxidant effect. So ellagic acid need be modified to ensure its antioxidant effect and improve its water-soluble oil-soluble, make its color pale. The experiment used the Valonia with cheap, raw materials extensive, rich in ellagic acid as subject. Then the Valonia was first hydrolyzed and next esterified, meanwhile the modified products were tested by antioxidation test and analyzed by IR, HPLC and NMR.The results show that: by comparing the effect of types of alcohol, catalyst, water-carrying agent on the Valonia modification, standard by hydroxyl radical scavenging, total phenol value, the yield, isopropyl alcohol is the best esterification agent, the best water-carrying agent is cyclohexane, the best catalyst is sodium bisulfate. The valonia extract hydrolysis-esterification modified condition were confined through by single-factor test and orthogonal experiment. Results revealed that the optimum modified conditions were as follow: according to the quality of valonic extract, alcohol consumption 8:1, the amount of catalyst 1:10, reaction temperature 80℃, reaction time 12h, the dosage of water entrainer 4: 5. Under this condition, the phenol content and the hydroxyl radical scavenging rate of modified product were highest, and the yelled was at 13%. Finally, The modified product was used in leather retanning process with different procedure and amount. The application process was optimized in Cr(VI) prevention at ageing of 80℃and UV irradiation conditions.The results show that: the best application amount of the modified product is 0.3%, the best adding time is 1 hour after adding fatliquoring. And adding in fatliquoring and 1 hour after fatliquoring both can make the Cr(VI) Content Less than 3mg/Kg.
|
PDF Full Text Request